The present invention relates in general to a laser-beam printer, and more particularly to an optical deflector which uses no mechanical moving parts and which is therefore improved in operating reliability and durability.
There is known a laser-beam printer that uses a laser beam to produce an electrostatic image-forming pattern a line at a time on the photoconductive surface of a drum. Such a laser printer employs an optical system which includes a laser light source for generating a laser beam, and an optical deflector for deflecting the laser beam over a predetermined angular range, so as to linearly sweep or scan the drum with the laser beam in the axial direction of the drum. An example of this type of laser printer is disclosed in Laid-Open Publication No. 51-8948 of Japanese Patent Application. Typically, the optical system utilizes a polygon mirror shown in FIG. 18, or a hologram scanner shown in FIG. 19.
In the optical system shown in a perspective view of FIG. 18, a laser beam emitted by a laser source 110 is transmitted through a collimator lens 112 and is incident upon a polygon mirror 114 which has a plurality of mirror surfaces. The laser beam reflected by the polygon mirror 114 is directed to a photoconductive drum 122 via a troidal lens 118 and an f.crclbar. lens 120. As the polygon mirror 114 is rotated by a motor 116, the laser beam reflected by the mirror 114 is deflected within a predetermined angular range to scan the surface of the photoconductive drum 122. In the optical system shown in a perspective view of FIG. 19, a laser beam produced by a laser source 124 reaches a hologram disc 130 via a cylindrical lens 126 and a stationary mirror 128. The hologram disc 130 has a diffraction grating 132, so that the laser beam reflected by the mirror 128 is passed through the grating 132, and reflected by a mirror 136 disposed over the disc 130. The beam reflected by the mirror 136 arrives at the surface of a photoconductive drum 142 via a spherical lens 138 and a cylindrical lens 140. As the hologram disc 130 is rotated by a motor 134, the angle of diffraction of the diffration grating 132 is varied, and the laser beam incident upon the drum 142 is accordingly deflected over a predetermined angular range, whereby the drum 142 is linearly swept by the laser beam.
As described above, the known optical systems use rotating parts such as a polygon mirror or hologram scanner to deflect a laser beam, motors to drive the rotating parts, and various lens and mirrors. Consequently, the optical systems tend to produce a relatively high level of operating noises, and are not necessarily satisfactory in their operating reliability and durability.